全文获取类型
收费全文 | 9787篇 |
免费 | 1426篇 |
国内免费 | 1318篇 |
专业分类
化学 | 5840篇 |
晶体学 | 119篇 |
力学 | 296篇 |
综合类 | 91篇 |
数学 | 773篇 |
物理学 | 1978篇 |
无线电 | 3434篇 |
出版年
2024年 | 21篇 |
2023年 | 136篇 |
2022年 | 205篇 |
2021年 | 326篇 |
2020年 | 320篇 |
2019年 | 320篇 |
2018年 | 286篇 |
2017年 | 321篇 |
2016年 | 445篇 |
2015年 | 497篇 |
2014年 | 554篇 |
2013年 | 702篇 |
2012年 | 797篇 |
2011年 | 833篇 |
2010年 | 684篇 |
2009年 | 634篇 |
2008年 | 716篇 |
2007年 | 670篇 |
2006年 | 623篇 |
2005年 | 574篇 |
2004年 | 440篇 |
2003年 | 424篇 |
2002年 | 456篇 |
2001年 | 352篇 |
2000年 | 235篇 |
1999年 | 212篇 |
1998年 | 140篇 |
1997年 | 117篇 |
1996年 | 72篇 |
1995年 | 66篇 |
1994年 | 39篇 |
1993年 | 41篇 |
1992年 | 44篇 |
1991年 | 45篇 |
1990年 | 28篇 |
1989年 | 21篇 |
1988年 | 28篇 |
1987年 | 18篇 |
1986年 | 20篇 |
1985年 | 19篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1975年 | 4篇 |
1957年 | 1篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 134 毫秒
941.
Byeong Hwa Yun Peter C. Dedon Nicholas E. Geacintov Vladimir Shafirovich 《Photochemistry and photobiology》2010,86(3):563-570
The photosensitized hole injection and guanine base damage phenomena have been investigated in the DNA sequence, 5′-d(CAT G 1 Py C G 2 TCCTAC) with a site-specifically positioned pyrene-like (Py) benzo[ a ]pyrene 7,8-diol 9,10-epoxide-derived N2 -guanine adduct (G1 Py ). Generation of the Py radical cation and subsequent hole injection into the DNA strand by a 355 nm nanosecond laser pulses (∼4 mJ cm−2) results in the transformation of G1 Py to the imidazolone derivative Iz1 Py and a novel G1 Py * photoproduct that has a mass larger by 16 Da (M+16) than the mass (M) of G1 Py . In addition, hole transfer and the irreversible oxidation of G2, followed by the formation of Iz2 was observed (Yun et al. [2007], J. Am. Chem. Soc., 129, 9321). Oxygen-18 and deuterium isotope labeling methods, in combination with an extensive analysis of the MS/MS fragmentation patterns of the individual dG Py * nucleoside adduct and other data show that dG Py * has an unusual structure with a ruptured cyclohexenyl ring with a carbonyl group at the rupture site and intact guanine and pyrenyl residues. The formation of this product competes with hole injection and thus diminishes the efficiency of oxidation of guanines within the oligonucleotide strand by at least 15% in comparison with that in the dG Py nucleoside adduct. 相似文献
942.
The direct patterning of poly(p‐methoxyphenylmethylsilane) (PMPMS) and poly(p‐methoxyphenylmethylgermane) (PMPMG) by interference lithography is reported. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
943.
Salvatore Abate Yun Gyong Ahn Tobias Kind Tommaso R. I. Cataldi Oliver Fiehn 《Rapid communications in mass spectrometry : RCM》2010,24(8):1172-1180
Many metabolomic applications use gas chromatography/mass spectrometry (GC/MS) under standard 70 eV electron ionization (EI) parameters. However, the abundance of molecular ions is often extremely low, impeding the calculation of elemental compositions for the identification of unknown compounds. On changing the beam‐steering voltage of the ion source, the relative abundances of molecular ions at 70 eV EI were increased up to ten‐fold for alkanes, fatty acid methyl esters and trimethylsilylated metabolites, concomitant with 2‐fold absolute increases in ion intensities. We have compared the abundance, mass accuracy and isotope ratio accuracy of molecular species in EI with those in chemical ionization (CI) with methane as reagent gas under high‐mass tuning. Thirty‐three peaks of a diverse set of trimethylsilylated metabolites were analyzed in triplicate, resulting in 342 ion species ([M+H]+, [M–CH3]+ for CI and [M]+ . , [M–CH3]+ . for EI). On average, CI yielded 8‐fold more intense molecular species than EI. Using internal recalibration, average mass errors of 1.8 ± 1.6 mm/z units and isotope ratio errors of 2.3 ± 2.0% (A+1/A ratio) and 1.7 ± 1.8% (A+2/A ratio) were obtained. When constraining lists of calculated elemental compositions by chemical and heuristic rules using the Seven Golden Rules algorithm and PubChem queries, the correct formula was retrieved as top hit in 60% of the cases and within the top‐3 hits in 80% of the cases. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
944.
In this paper, a phosphorescent Cu(I) complex of [Cu(POP)(ECI-Phen)]BF4, where POP=bis[2-(diphenylphosphino)phenyl]ether, and ECI-Phen=1-ethyl-2-(N-ethyl-carbazole-yl-4-)imidazo[4,5-f]1,10-phenanthroline, is incorporated into a polystyrene matrix of polystyrene (PS) to form microfibers membranes. The possibility of using the resulted composite microfibrous membranes as an optical oxygen sensor is explored. Good linearity and short response time are obtained with a sensitivity of 9.8. These results suggest that phosphorescent [Cu(POP)(ECI-Phen)]BF4 is a promising candidate for oxygen-sensors and PS is an excellent matrix for oxygen sensing material because it owns a large surface-area-to-volume ratio and can supply a homogeneous matrix for probe molecules. Further analysis suggests that the molecular structure of diamine ligand in Cu(I) complexes is critical for sensitivity due to the characteristic electronic structure of excited state Cu(I) complexes. 相似文献
945.
Deculsnea./al..gesiiFranch.IsatraditionalChinesemedicinalplantwidely'distributedinChina.usedasananti-rheumaticandalltitussivedrugforalongtimeinChinesefolkmedicine'.Itsmethanolextractshowedantitunlouracidityif71)if)oagainstS:sl},HepaandEhrlichcellsanditsphytochemicalstudiesonsaponinshasreported'.Asacontinuationofthestudiesonthisplant,wepresentthespectralandchemicalevidenceofthestructureofanewdecaisosideF.Thedriedstems(2.23kg)werepercolated\vith95%EtOH.Afterremovalofthepercolatesb}evaporat… 相似文献
946.
947.
Superparamagnetic pH‐sensitive multilayer hybrid hollow microspheres for targeted controlled release
Bin Mu Peng Liu Yun Dong Chunyin Lu Xueli Wu 《Journal of polymer science. Part A, Polymer chemistry》2010,48(14):3135-3144
The superparamagnetic multilayer hybrid hollow microspheres have been fabricated using the layer‐by‐layer assembly technique by the electrostatic interaction between the polyelectrolyte cation chitosan (CS) and the hybrid anion citrate modified ferroferric oxide nanoparticles (Fe3O4‐CA) onto the sacrificial polystyrene sulfonate microspheres templates after etching the templates by dialysis. The saturation magnetization and magnetite contents of the superparamagnetic multilayer hybrid hollow microspheres were 32.46 emu/g and 51.3%, respectively. The hybrid hollow microspheres showed pH‐sensitive characteristics. The adsorption and release of the basic dye (methylene blue) were applied to investigate the interaction between the amino groups of CS and the carboxyl groups of the Fe3O4‐CA nanoparticles in different pH media. The superparamagnetic pH‐sensitive multilayer hybrid hollow microspheres are expected to be used for the targeted controlled release of drugs or in diagnostics. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3135–3144, 2010 相似文献
948.
Hong Ping HE Yun ZOU Yue Mao SHEN Xiao Yan HAO Guo Yin ZUO Xiao Jiang HAO * Laboratory of Phytochemistry Kunming Institute of Botany Chinese Academy of Sciences Kunming . Department of Medicinal Chemistry Guiyang Medical College 《中国化学快报》2001,(7)
Micromelum integerrimum (Rutaceae) is a shrub growing in Yunnan Province. Leaves and barks of this plant have been used for the treatment of dysentery, and arthritis1. Previous researches revealed that this plant contained coumarins2-8. Micromelin, the main chemical constituent of this plant, showed significant inhibition against leukaemia P-388 cell line (T/C 149% at 10 mg/kg) but was inactive in lower doses, and also showed significant activity (T/C 228% at 1.25 mg/kg) against Lewis lung… 相似文献
949.
Sinceexcellentantitumoractivityanduniquemechanismofactionofpaclitaxel(taxol@)werediscovered,chemicalstudiesonTaxusspecieshavebeenextensivelycarriedoutl'2.About300taxoidshavebeenisolatedfromGeneraTaxusandAustrotaxusplants'.Insearchforbioactivetaxoidsa... 相似文献
950.
随着科技的飞速发展 ,计算机在化学化工中的应用越来越广泛深入 ,其中一个重要方面就是用计算机来绘制各种化学、化工的专业图形。在众多的化学制图软件中 ,ISIS Draw以其完善的制绘多种化学分子结构式、化学反应方程式及化工流程图的功能 ,引起了广大化学工作者的注意。ISIS Draw是 1 990年由MDL公司开发的一个化学制图软件 ,它的较新版本为 2 2 1 ,是 1 999年4月升级后的第五代产品。该软件可从下列网址方便地免费下载 :http : www .mdli.com cgi dynamic downloadsecthtm… 相似文献